For example, Patent Document 1 (Japanese Patent No. 4267042, for example) discloses an ignition coil for an internal combustion engine, which includes a main body that has a primary coil and a secondary coil magnetically coupled to each other, and a plug boot for connecting the main body and a spark plug.
The plug boot has a tubular shape, and is flexible and electrically insulating. A coil spring for electrically connecting the secondary coil and the spark plug is inserted and disposed inside the plug boot.
The plug boot has a role of electrically insulating the coil spring inserted inside the plug boot and the outside of the plug boot.
Here, the ignition coil may be used by inserting a plug boot into a bent plug hole.
In view of this, the ignition coil disclosed in Patent Document 1 has a thin-walled portion formed in a part of the plug boot in an axial direction. The thin-walled portion is formed thinner than the other portions by recessing an outer peripheral surface of the plug boot toward an inner peripheral side thereof.
Accordingly, the ignition coil disclosed in Patent Document 1 aims to allow easy bending of the plug boot in the thin-walled portion and easy insertion into the bent plug hole.
However, the withstand voltage of the plug boot is determined according to the thickness of the plug boot. That is, the lower the thickness of the plug boot, the lower the withstand voltage.
Therefore, in the ignition coil disclosed in Patent Document 1, it is required to have a thickness at least to the thin-walled portion greater than or equal to the thickness that can ensure electrical insulation between the inside and the outside thereof.
Furthermore, a portion that is stretched by bending occurs in the thin-walled portion when the plug boot is bent at the thin-walled portion.
This portion becomes smaller in thickness than when it is in a free state where the plug boot is not bent.
Therefore, in the ignition coil bent at the thin portion, considering that the thickness of the thin portion becomes thinner after the plug boot is bent at the thin portion, a sufficient thickness may not be obtained at the thin portion for obtaining insulation.
Along with this, portions other than the thin wall portion of the plug boot necessarily have a wall thickness sufficiently thicker than the thickness required for securing electrical insulation between the inside and the outside thereof. This leads to an increase in the cost and size of the plug boot.